1. Field of the Invention
This invention generally relates to a composition for an optical material. More specifically, it relates to a composition for an optical material which has a refractive index and weatherability equivalent to those of diethylene glycol bis(allyl carbonate), which has been used for eyeglass lenses and other optical applications (and which will be hereinafter referred to as the "above-mentioned compound"), has a low shrinkage ratio at the time of polymerization, and moreover has excellent impact strength.
2. Description of the Related Art
Organic glass is lighter in weight than inorganic glass. Accordingly, organic glass consisting of the polymer of the above-mentioned compound has been used for eyeglass lenses. However, the volume shrinkage ratio of the above-mentioned compound is as great as 14% during casting polymerization. For this reason, there remains the problem that cracks are likely to occur in molded articles when molding is carried out within a short period of time.
To avoid this problem, industrial production of organic glass has been carried out by partially polymerizing the above-mentioned compound so as to increase its viscosity, and then effecting molding slowly at a low temperature in the course of more than 12 hours using a radical polymerization initiator of a low temperature decomposition type having a high activity such as diisopropylperoxy dicarbonate.
However, this method has extremely low productivity and is not economical. Therefore, Japanese Unexamined Patent Publication (Kokai) No. 52-60892 proposes a method which combines an aliphatic ethylenically unsaturated polyester with an unsaturated monomer, as a method of reducing a shrinkage ratio. However, since fumaric acid and maleic acid are used as the ethylenically unsaturated group in this case, there remains the problem that a radical polymerization rate becomes so fast that control during polymerization becomes difficult and distortion becomes more likely to occur during this process.
On the other hand, U.S. patent specification No. 4,217,433 proposes a method which dissolves a copolymer of methyl methacrylate and allyl methacrylate in the above-mentioned compound in order to reduce the shrinkage ratio at the time of polymerization. However, this method is not yet free from the problems that the copolymer as the starting material is expensive, and if the proportion of allyl methacrylate of the copolymer is high, a cross-linkage density of the molded article becomes so high that the resulting product becomes brittle, and if the proportion of methyl methacrylate is so high, transparency of the cured product drops.
An allyl ester which has an allyl ester group at the terminal, the main molecular chain of which is derived from a polyvalent aromatic or saturated aliphatic carboxylic acid and a polyhydric saturated alcohol, and which has a structure represented by the following formula, is also knowns: EQU CH.sub.2 =CHCH.sub.2 O(CORCOOBO).sub.n CORCOOCH.sub.2 CH=CH.sub.2
wherein, R represents an organic residue derived from a polyvalent aromatic or saturated aliphatic carboxylic acid, B represents an organic residue derived from a polyhydric saturated alcohol, and n is an integer.
In this case, if terephthalic acid or isophthalic acid is used as the polyvalent saturated aromatic carboxylic acid, a refractive index is relatively high and impact resistance is also excellent. However, weatherability in this case is by far lower than when the above-mentioned compound is used. Therefore, this glass does not provide satisfactory performance as organic glass which is assumed to be used as a lens for outdoor use without any correction factor such as glass for sunglasses.